The present disclosure relates generally to hydroxyapatite (HA)-based materials coated with modular biologically active molecules such as modular peptides. Particularly preferred modular biologically active molecules may include modular cytokines, growth factors, hormones, nucleic acids, and fragments thereof. Of particular importance in this disclosure are modular growth factors having improved non-covalent binding to the surface of the HA-based materials and being capable of initiating osteogenesis, angiogenesis, and/or osteogenic differentiation.
Natural proteins often contain at least two functional domains, which are linked together to form one multi-functional protein molecule. Specifically, these proteins are capable of activating cell surface receptors, and also binding with high affinity and specificity to natural extracellular matrices (ECMs). To achieve these diverse functions, a strategy commonly employed by nature involves creating modular proteins, in which distinct domains within a single protein are designed to enable either cell signaling or ECM binding. For example, the bone ECM protein osteocalcin (OCN) binds to HA, the major mineral component in the ECM of bony tissues, with high affinity via an N-terminal domain, and also plays a critical role in regulating bone matrix formation via a C-terminal domain.
The mechanisms that enable the binding of signaling molecules to ECM in nature can potentially be extended to synthetic biomaterials as well. For example, a recent study indicated that it is possible to mimic nature's modular cell adhesion proteins (e.g. OCN, bone sialoprotein (BSP)) by engineering synthetic modular peptide molecules that bind to synthetic HA, yet remain capable of affecting cell adhesion. This modular design approach has been used to promote cell adhesion to natural and synthetic HA-based materials, which are now used in a wide range of common clinical orthopedic applications. However, previous studies have not been able to actively induce new bone formation by bone precursor cells, nor are they able to induce differentiation of stem cells into bone-forming cells.
Musculoskeletal conditions represent an average of 3% of the gross domestic product of developed countries, consuming an estimated $254 billion annually in the United States. Bone and joint diseases account for half of all chronic conditions in people over the age of 50, and the predicted doubling of this age group's population by 2020 suggests that the tremendous need for novel bone repair and replacement therapies will continue to grow rapidly. Emerging therapeutic approaches have focused on delivering growth factor molecules to skeletal defects, as these molecules are capable of actively inducing new bone formation. However, growth factor delivery strategies often result in sub-optimal delivery kinetics, and are difficult to incorporate into standard clinical procedures. These limitations complicate clinical translation of growth factor delivery in orthopedic applications.
Accordingly, there is a need in the art for modular growth factors that can be engineered to bind strongly to HA and HA-based materials, thereby forming a biologically active “molecular coating” with controllable characteristics. Specifically, it would be advantageous if the modular growth factor had two functional units, similar to natural proteins: a HA-binding sequence to allow for improved binding to the surfaces of HA and HA-based materials; and a biomolecule-derived sequence inspired by natural biologically active molecules such as bone morphogenetic protein-2 (BMP-2) and vascular endothelial growth factor (VEGF). These modular growth factors may be broadly applicable in orthopedics, as HA is among the most commonly used materials in orthopedic applications, including total joint replacements, trauma, and fracture healing.